There are many known techniques for suppressing electromagnetic interference (EMI) from a system, such as computer system to contain or diminish stray noise signals. Systems such as computer systems typically need to comply to an electromagnetic compliance (EMC) standard which defines limits to levels of stray EMI noise signals. Thus, the design goal of an EMC solution is to design a system with stray EMI noise signal levels below the EMC standard limit while minimizing the cost of compliance.
A system, such as a computer system, is typically designed with distinct domains to separate domains having energetic EMI noise sources from more sensitive domains having regions with less robust EMI shielding to an outside environment. Example more sensitive domains of a computer system include regions containing input/output (I/O) card cages, system I/O, and peripheral devices because it is particularly difficult to suppress EMI conducted from these regions. In high-performance computer systems, the most energetic EMI noise sources typically include processors, memory, and coupling circuitry which couples processors to other processors, memory, or I/O devices. The stray EMI noise signals produced by the processors, memory, and coupling circuitry conducts to I/O or peripheral domains from which the stray EMI noise radiates from cables or directly from user accessible devices. I/O boards, disk drives, or core I/O devices are typically not significant contributors to the total direct radiated EMI noise produced by the computer system.
A robust EMC computer system design which partitions the energetic processor domain from the I/O and peripheral domains typically blocks most of the direct radiated EMI noise signals from entering the susceptible I/O and peripheral domains. Unfortunately, DMI noise signals also conduct through interconnecting electrical paths between the partitioned domains as common mode energy EMI. EMI noise is also typically conducted through power paths and ground paths. Any conducted EMI noise contaminates an otherwise quiet I/O or peripheral domain leading to leakage to an outside environment.
One conventional EMC system design disposes ferrite slabs over semiconductor integrated circuit (IC) chips to suppress direct radiated EMI signals from entering susceptible domains. This solution, however, does not prevent the conduction of common mode energy EMI through interconnecting electrical paths between domains.
One conventional EMC system design to limit excessive cable radiated common mode energy EMI resulting from cross domain EMI noise contamination includes ferrite attenuators or suppressors mounted around cables approximate to where the cables exit the system. The ferrite attenuators surrounding the cables elevate EMI noise source impedance via electrical losses occurring in the ferrite. As a result, the efficiency of the propagation of common mode energy EMI noise signals to free space is reduced. Since this design technique does not prevent EMI noise contamination of an I/O card cage, each cable exiting the system is required to have a ferrite attenuator mounted on the cable approximate the exit point from the system. In systems, such as computer systems, where many cables exit the system, the cost of this solution is excessive. In addition, the effectiveness of this technique is reduced if cable placement is changed or if the ferrite attenuator is improperly located on the cable.
For reasons stated above and for other reason presented in greater detail in the Description of the Preferred Embodiments section of the present specification, a system, such as a computer system, is desired which better suppresses common mode energy EMI noise which propagates through interconnecting electrical paths, power paths, and ground paths between domains and eventually radiates to free space. In addition, a system, such as a computer system, is desired which effectively suppresses common mode energy EMI at a low cost.